The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the inventors hereof, to the extent the work is described in this background section, as well as aspects of the description that does not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted to be prior art against the present disclosure.
In typical MUMIMO systems, an access point applies a precoding matrix to a single downlink (DL) MUMIMO packet and transmits the data to all of the receiving stations simultaneously. In such systems, the precoding matrix is computed by the access point using an explicit sounding protocol. In particular, the access point transmits a sounding packet to multiple receiving stations in the format of a non-data packet (NDP). Each receiving station computes a channel estimate and generates a feedback matrix based on the channel estimate. The feedback matrix represents a gain and phase across all available subcarriers of each receiving antenna of the station. Each receiving station then sends its respective feedback matrix back to the access point either sequentially (in the case of 802.11ac protocol) or simultaneously using an uplink-MU packet (in the case of 802.11ax). The access point combines the feedback matrices received from each receiving station and computes the precoding matrix for use in downlink data transmissions.
In these systems, the receiving stations compute their respective feedback matrix based on the number of receiver antennas at the station and the number of transmitter antennas used by the access point for sounding. For example, if the access point transmits the NDP from 4 transmitter antennas and a station receives it with 2 receiver antennas, a 4×2 channel matrix may be calculated for each subcarrier. In some cases, the access point may have more antennas (e.g., 8 antennas) than the number of antennas supported by receiving stations (e.g., 4 antennas). When the access point has more antennas than what is supported by the receiving stations, additional beamforming phase and gain for downlink transmissions using the additional antennas cannot be determined by explicit sounding. This is because, in explicit sounding, the feedback matrices that are fed back from the receiving stations represent the channel estimate for only the antennas the respective stations support (e.g., 4 antennas) and not all the antennas (e.g., 8 antennas) of the access point. This results in the additional antennas of the access point being wasted in downlink transmissions as the access point is forced to act as a four antenna (4×4) access point rather than an eight antenna (8×8) access point.